Run travis tests on GHC 7.6, 7.8, 7.10

[etrepum]

To ensure that the tests and reference apps run without errors or warnings, we should automate tests on GHC 7.6 (because people use Linux distros with old versions of Haskell), GHC 7.8 (not everyone is on the bleeding edge) and GHC 7.10.

Since it's not feasible to use Haskell Platform from Travis we need to curate a list of packages to cabal install first.

• https://github.com/ekmett/lens/blob/master/.travis.yml is an example of a project that successfully tests multiple GHC versions

We may want to investigate halcyon and/or stack for faster builds. Note that stack does not support GHC 7.6, only 7.8 and 7.10. We could change the installation instructions and drop support for GHC 7.6, but that may be too high of a cost.

[etrepum]

Constraints:

• Tests should be easy and quick to run for contributors (probably with just the installed GHC version). This rules out requiring halcyon for all test runs (it's not easy to install except on Linux).
• Travis must run tests on 7.6, 7.8, and 7.10 to support the wide range of Haskell users that we already have. Don't want to alienate the folks on Linux using their distro's 7.6 build. Travis needs to run tests on 7.6 (as it does now) since I don't expect contributors to keep a 7.6 install around to run the tests themselves. This rules out requiring stack everywhere because it doesn't support 7.6.
• Shouldn't require a Haskell Platform install. We'll want to enumerate the set of packages that we actually depend on somewhere (cabal file is probably appropriate).